Electric power control system



Nov. 29, 1960 J, J, Ew ETAL 2,962,598

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Their Afiorney I Nov. 29, 1960 J. J. LAREW ETAL ELECTRIC POWER CONTROL SYSTEM 7 Sheets-Sheet 2 Filed May 28, 1956' SYSTEM POWER LINE ELECTRONIC POSITIONING UNIT LOAD PRO-g PORTIONING SYSTEM POWER LINE ELECTRONIC POSITIONING UNIT LOAD PRO- PORTIONING CARRIER CURRENT RECEIVER FREQUENCY MULTIPLIER SERVO AMPLIFIER SYJT EM POWER LIN E BALANCING AMPLIFIER FUNCTION GENERATOR lfivemors John J. Lorew Wesley S. BurI BiIIy R. She 0rd byfMmX: $1

Their AHorney L A M N N vm. m 5 T5 m M U W m 6 M 0 w w 5 M 7/ M BA 2 R wA F 4 a m 7 MN E F6 Nov. 29, 1960 J. J. LAREW ET AL 2,962,598

ELECTRIC POWER CONTROL SYSTEM Filed May 28, 1956 7 Sheets-Sheet 3 Fig.3

lnventors John J. Lurew Wesley S. Buri BiHy R. Shepard Their Arrorney Nov. 29, 1960 J. J. LAREW ET AL ELECTRIC POWER CONTROL SYSTEM 7 Sheets-Sheet 4 Filed May 28, 1956 dekzzauwa lnvenrors John J. Lurew Wesley S. Bur: Billy R. S e by. 4 Their AH'or paid ney

Nov. 29, 1960 J. J. LAREW ET AL 2,962,598

ELECTRIC POWER CONTROL SYSTEM FIZLlBd May 28, 1956 7 Sheets-Sheet 5 205b 206b F22 Fig.5

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lnvenhrs Their A+ orney Nov. 29, 1960 J. J. LAREW ET AL 2,962,598

ELECTRIC POWER CONTROL SYSTEM Filed May 28, 1956 7 Sheets-Sheet 6 Fig. ll

lnven+orsz John J Lcrew Wesley S. Bur! Billy R.Shepord Their Arrorney Nov. 29, 1960 J. J. LAREW ETAL ELECTRIC POWER CONTROL SYSTEM Filed May 28, 1956 '7 SheetsSheet 7 Invenfors: John J. Larew Wesley S. Burr Bllly R. Shepard by. Their A: *orney rates ELECTRIC POWER CONTROL SYSTEM Filed May 28, 1956, Ser. No. 587,561

33 Claims. (Cl. 290--4) This invention relates to an electric power control system, and more particularly to apparatus for controlling most economically the output of a power system which comprises a plurality of interconnected generators and generating stations, which may be connected to other power systems by one or more tie lines, in response to changes in load and frequency. The present application is a continuation-in-part of copending application Serial No. 395,021 filed November 30, 1953 and now abancloned. The present invention, as well as the invention of said copending application, constitutes an improvement on the electric power control system which is the subject of copending application Serial No. 395,022, filed November 30, 1953, by Edward E. Lynch and John J. Larew, now Patent 2,824,240, issued February 18, 1958, and assigned to the same assignee as the present invention.

It is common practice for neighboring power systems to be interconnected by one or more tie lines over which an interchange of power is made according to preselected schedules. It is necessary, of course, that the tie line interchange be held at the previously scheduled values, and the frequency also held at its desired value.

Generally, each power system includes a plurality of generating stations, each of which contains a plurality of generators. It is known that if a plot is made of fuel input to drive a generator as a function of output power for the generator, the resulting plot, which is known as a fuel-input curve, is a curve and not a straight line and it varies from generator to generator. From this curve, another plot may be made of the slope of the fuel-input curve versus power output, which is known as an incremental fuel-rate curve. The incremental fuel rate may be converted to incremental cost by multiplying the incremental fuel rate by the fuel cost. It is known that, where all generator units are interconnected to supply the same system, and transmission losses are neglected, generally the minimum system fuel cost for a given load is obtained when all generator units are operated at the same incremental fuel cost.

In addition, the generators or generating stations having various incremental fuel costs are generally located at different distances from the load center and so there are various transmission losses which must be taken into account. For example, it must be determined whether or not it is cheaper to send low-cost power over a long distance than to send high-cost power over a short distance. The transmission losses which any given station must suffer in transmitting its generated power to a distribution point are generally accounted for in computations involving power distribution by a penalty factor assigned to the transmitting station. Therefore, a control system for controlling the power output of these generating stations should permit manual or automatic adjustment of the output to permit loading in accordance with incremental fuel cost and the penalty factor; that is, the output of each generator or station must be manually or automatically adjustable for maximum economy.

It is a primary object of the present invention to proatent I? vide apparatus for automatically controlling the generation or output of a plurality of generators and generating stations that is capable of loading for maximum economy, holding tie line power interchange to previously arranged schedules automatically and, simultaneously, holding the system frequency at a predetermined value.

Another object of the invention is to provide a system wherein costly duplication of control equipment at each station is eliminated by controlling the system from a central control station that requires only one control signal communication channel from the central control station to each controlled generating station.

Another object is to provide a load-frequency control system wherein an identical signal representing change in incremental cost of generated power may be sent to all generating stations, where it is converted to signals representing desired power output from the individual generators for this incremental cost, and the power output of the generators is varied in accordance with the latter signals so that the incremental cost of generated power is the same for all generators.

Another object is to provide a control system wherein a change in power output is made quickly in response to a change in area requirement, and the output of the various generators is then reapportioned at a slower rate in accordance with economic loading considerations.

Another object of the invention is to provide a power control system which is capable of effectively controlling the generation of a local power area when the local area is not interconnected with a remote area or, if a local and remote area are interconnected, is capable of holding the power interchange at a prescheduled value without regard for system frequency or the system frequency at its predetermined value without regard for power interchange.

Another object is to provide a tie line load-frequency control system which does not require readjustment when a generator or station ceases operation or starts operation to continue operation of the controlled units at equal incremental costs.

A further object of the invention is to provide a system which is as simple and inexpensive in first cost and maintenance as is consistent with performance of the required functions.

The basic operation of a tie line load-frequency control system can be shown by considering two areas, A and B, interconnected by a tie line and both having their own generation and loads. Economic and other considerations would dictate the most desirable load to send over the tie line, and this could be redetermined as often as required. Generally, the power interchange is rescheduled daily to provide loading for maximum economy and is readjusted at various times during the date of this schedule. In addition to holding the tie line load to the prescheduled value, it is essential that the frequency of the generation in both areas be held at the desired value, which in present day operations is usually 60 cycles per second.

It is known that tie line load and frequency can both be held to the desired values by proper manipulation of the generator output at A and B, in spite of load changes at either A or B. For example, assume that the condition exists wherein the loads at A and B and the scheduled tie line load are exactly supplied. Then, if a reduction in load occurs in area A, it results in an excess of generation at A over that required, which tends to increase both the frequency of generation and the tie line power flo'w from A to B. Similarly, an increase in load at A results in decreased frequency and decreased tie line power flow from A to B. However, when the load at B decreases, the frequency increases, but the tie line load from A to B decreases. Similarly, when the load at B increases, the frequency decreases, and the tie line load from A to B increases. Therefore, it is apparent that an increase in both frequency and the tie line power fiow from A to B, or a decrease in both frequency and tie line power flow from A to B, indicates a load change in area A. Similarly, when the frequency and tie line power flow or load change in opposite directions, it indicates a load change in area B. Therefore, by knowing both the change in frequency and change in tie line load, it can be determined whether the load has changed in area A or area B, and the proper adjustments can be made in the generation for these two areas. These facts indicate that for the tie line load-frequency control, either automatic or manual, at least two primary detectors are required; one which detects frequency and one which detects load.

It is apparent that to restore the system to predetermined or scheduled conditions after the load changes in both areas, corrective action is required at both areas A and B; and if area A properly increases or decreases its generation exactly to supply increases or decreases in load at A, area B will be required to take care of only if its own load changes by corresponding generation changes at B. The system of the present invention is limited to corrective action in only one such area; that is, a control system is required in area A to take care of changes in load and frequency in area A, and another system is required in area B to take care of changes in load and frequency in area B. Thus, a control initiating signal which is a function of frequency divergence and tie line load divergence is required in order to accomplish this desired control automatically, with the system comprising the present invention.

A load-frequency power control system constructed in accordance with the invention comprises essentially two parts, detector means and control signal producing means located at a central control station and control signal responsive means located at each of the generating stations comprising the controlled power network. The detector means function to produce a signal Af proportional to the deviation of the actual system frequency from a standard frequency and to produce a signal Aw proportional to the deviation of the actual tie line load from its scheduled value. The control signal producing means combines the A and Aw signals into a control signal which may be considered as representing either area power requirement or a change in incremental cost of generated power. The control signal is then transmitted by conventional means to the individual generating stations.

Each generating station embodies control signal responsive means which utilize the control signal to vary the power outputs of the various generators within the station. The signal responsive means includes means for selecting all or a fractional part of the control signal transmitted thereto to be supplied to individual controls for each generator, and means for each generator for converting the control signal to a signal representing desired power output for that particular generator. The generation of each generator is then varied in accordance with the corresponding desired power output signal.

The control signal responsive means at each generating station comprises two channels, both of which receive the control signal from the central control station. One c annel acts to change the power output of the generating station relatively quickly in accordance with the area requirement represented by the control signal, and other channel acts more slowly to reapnortion the outputs of the variou generators comprising the station in accordance with incremental fuel cost differences between the generators.

The frequency deviat on signal Af and the load deviation si nal Aw are combined in the detector means in such a manner t at if t e deviations are due to a change in lo d in area B, At and Aw tend to cancel each other and no c ange is m de in the control signal to the stations of area A. If the change in load which causes the change in tie line power interchange and the change in system frequency has occurred in area A, which is taken to be the area which is controlled by the system of the invention, a corrective signal will be applied to the governors which is continuously present and proportional to the deviation from schedule.

For a better understanding of the invention, together with further objects and advantages thereof, reference is made to the following description taken in conjunction with the accompanying drawings, in which:

Fig. 1 is a schematic block diagram of one embodiment of detector means and control signal producing means located at a central control station;

Fig. 2 is a schematic block diagram of the control signal responsive means located at each generating station;

Fig. 3 is a schematic diagram of a tie line load controller shown in block form in Fig. 1;

Figs. 4 and 4a are schematic diagrams of a servo amplifier that may be embodied in the apparatus shown in Fig. 1;

Fig. 5 is a schematic diagram of a function generator shown in block form in Fig. 2;

Figs. 6 and 7 are graphs useful in understanding the operation of the function generator of Fig. 5;

Fig. 8 is a schematic diagram of a balancing amplifier shown in block form in Fig. 2;

Fig. 9 is a schematic diagram of an electronic position-unit shown in block form in Fig. 2;

Fig. 10 is a schematic block diagram of a modification of the detector means and control signal producing means installed at a central control station;

Fig. 11 is a schematic diagram of the modifying circuit shown in block form in Fig. 10, and

Fig. 12 is a schematic diagram of the proportional controller shown in block form in Fig. 10.

The power control system of this invention may be used to control the generation of a local power area, which is interconnected with one or more remote power areas by tie lines, in response to changes in the tie line load and frequency. Also, the control system may be made responsive to tie line load changes only or to system frequency changes only. However, for purposes of explanation, the more involved system, which is responsive to both tie line load and system frequency changes, is illustrated and will be described hereafter.

The terms frequency and phase are used interchangeably throughout this specification, as are the terms frequency difference and phase difference and the interchangeable terms are taken to have the same meanings.

The tie line load-frequency control system of the invention embodies two primary detectors, one for detecting deviation of the system frequency from a desired value, and one for detecting the tie line load deviation from its prescheduled value, both forming part of the central control station equipment. Referring now to the embodiment of that equipment shown in Fig. 1, the means for detecting the deviation of the tie line load from the pre-scheduled value comprises a tie line load controller 20 and a differential selsyn 21. One input to the tie line load controller 20 is from a conventional telemeter receiver or the like (not shown), such as is available commercially, which provides to the load controller a signal w proportional to the actual tie line load. The tie line load controller 20 is essentially a servo-mechanism. and any one of numerous known devices may be so emploved. One servomechanism, which i known to be suitable for this use, is shown and described in US. Patent 2,753,505 issued July 3, I956, on copending appl cation Serial No. 395,117, filed November 30, 1953, by I. J. Larew and K. N. Burnett, and as igned to the same assignee as the present invent on. A schematic diagram of th t device is s own in Fig. 3 and will be described hereafter. However, the present invention is not limited to the use of any one particular device for this purpose. Briefly, the tie line load controller ,20 compares the signal w, received from the telemeter receiver with another signal w which is produced with- .in the tie line load controller and whose value may be made proportional to the pre-scheduled value of tie line load, and produces rotation of an output shaft 22 at a speed proportional to the amplitude difference Aw between the two signals and in adirection determined by the polarity of the difference.

The output shaft 22 is connected ,to therotor of the differential selsyn 21, whose three p'hase stator winding is energized from the system power line, having a frequency f As is well known, when the rotor of a differential selsyn is rotated, the frequency of the voltage induced in its three-phase winding is equal to the frequency of the voltage which energizes the stator winding plus or minus the speed of rotation of the rotor. For example, if the stator winding of a differential selsyn is energized .by a 60-cycle per second voltage and the rotor is turned at revolutions per second, the frequency of the voltage induced in the three-phase rotor windings will be either 55 cycles per second or 65 cycles per second, depending on the direction of rotation of the rotor. In the present instance, the shaft 22 which drives the rotor of the differential selsyn 21 is rotated at a speed kAw which is proportional to the tie line load deviation from the scheduled value, where Aw is the deviation and k is a proportioning constant. Therefore, the frequency of the induced voltage on the rotor of selsyn 21 is (f -i khw), where kAw may be either positive or negative, depending on the direction of rotation of shaft 22 and the selsyn rotor.

The voltage induced on the three-phase rotor of differential selsyn 21, having a frequency (f -i-kAw), is supplied to one input of a servo amplifier 23, which, with a frequency standard 24, comprises the means for detecting the system frequency deviation from the standard or desired frequency. The output of the frequency standard 24 is supplied to a second input of the servo amplifier 23. The frequency standard 24 may be of any conventional well-known type, such as an oscillator whose frequency is controlled by a tuning fork or a crystal, and its principal requirements are that it provide an output signal of sulficient amplitude to drive the servo amplifier 23 and of a frequency i which is constant to the degree required by public utility power generation systems. The servo amplifier 23 may be one of several known designs. One servo amplifier, which is known to be suitable for this use, is described in copending application Serial No. 395,119 filed November 30, 1953 by J. J. Larew and C. E. James, now Patent No. 2,856,523, issued October 14, 1958, and assigned to the same assignee as the present invention. The schematic diagram of that amplifier is shown in Figs. 4 and 4a and will be described hereafter. However, the present invention is not limited to the use of any particular device and any servo amplifier that functions in the described manner may be employed. Briefly, the servo amplifier 23 compares the frequencies or phases of the two input signals and produces rotation of an output shaft 25 at a speed proportional to the frequency or phase difference between the two signals and in a direction determined by the polarity of the difference.

As was previously mentioned, the signal supplied to the servo amplifier 23 from the differential selsyn 21 has a frequency (f -l-kuw), and the signal supplied thereto from the frequency standard 24 has a frequency f,. The frequency f may be defined as f =f +Af, where A) may be positive or negative. Therefore, when the two input signals are compared in the servo amplifier and the shaft 25 caused to rotate at a speed proportional to the frequency difference, the shaft rotates at a speed proportional to (Af-i-kAw). Thus if the tie line load controller 20 is adjusted to rotate out-put shaft 22 and the rotor of differential selsyn 21 in a direction to add to the stator energizing frequency f, when the actual tie line load .is

greater than the scheduled load and to subtract from 1,,

when the actual load is less than scheduled, M and kAw will be arithmetically added together when the tie line load and system frequency both vary in the same direction from their respective standards. As was previously mentioned, system frequency and actual tie line load both increase or both decrease with the load changes in the local area, whose generation is being controlled by the system, when the line power is considered as flowing from area A to area B.

On the other hand,'when the load changes in the remote area,'the system frequency and tie line load vary in opposite directions, and A and kAw tend to cancel each other and cause no change in generation in the local area. Since this effect is desired, the constant k, by which Aw is multiplied, is adjusted by varying the gain of the tie line load controller 20 so that A and kAw just cancel each other, when the changes in system frequency and tie line load are due entirely to a load change in the remote area. The proper value of k may be found easily by experimentation. Of course, a constant k may be applied to the frequency deviation signal A) rather than to the load deviation signal Aw, so long as its value is such as to cause the two deviation signals to cancel each other when they are caused by lead changes in the remote area.

It is again pointed out that various means may be employed to cause rotation of a shaft, such as the shaft 25, proportional to the algebraic sum of the two deviation signals, and the control system of the invention is not limited to any particular methods or apparatus for producing the deviation signals.

The shaft 25 is connected to the rotor of a differential selsyn 26, and the three-phase stator winding of the selsyn is energized from the system power line having frequency f,,. Therefore, the frequency of the voltage induced on the three-phase rotor winding of differential selsyn 26 is f -l-(Af-l-kAw), which is the fundamental control signal to be sent to the generating stations. It is apparent that deviation of the control signal frequency from the system frequency is proportional to the area power requirement and the integral of that frequency difference is a function of system incremental cost of generated power or of system load.

In order to provide a visual indication of system conditions to an operator at the central control station, the shaft 25 is also connected through suitable gearing to the movable contact pick-off arm of a potentiometer 27. The potentiometer 27 is connected across a source of unidirectional potential 28 and a voltmeter 30 connected to the pick-off arm of the potentiometer provides an indication of the position of the movable pick-off arm.

The potentiometer 27 serves to integrate the output of the servo amplifier 23 (the area requirement or change in incremental cost signal) and the position of the potentiometer pick-off contact indicates system load or incremental cost. Thus, the scale of voltmeter 30 may be calibrated in terms of either of those quantities; it will be assumed for purposes of the following description that the voltmeter 30 indicates incremental cost of generated pow er. Of course, the incremental cost indication may be obtained through means other than a direct current en ergized potentiometer.

The rotor windings of differential selsyn 26 on which the control signal appears, are connected to the stator windings of a control transformer selsyn 31 whose rotor is connected to a reversible motor 32. The selsyn 31 and the motor 32 form part of a penalty factor unit 33, one such unit being provided at the central control station for each controlled generating station. The motor 32 may be caused to rotate by manually closing a switch (not shown) to connect the motor to a suitable power source. When motor 32 is energized and the rotor of control transformer selsyn 31 rotated, the frequency of the system control signal induced in the rotor windings is 7 increased or decreased from the frequency of the signal which energizes the stator winding of the selsyn. In this case, the effect of rotating the rotor of the selsyn is to add to or subtract from the system control signal frequency, which might be done in order to reapportion the load between the various stations of the network.

As is well known to those skilled in the power transmission art, definite losses occur in transmitting power from a generating station to a load. In order for optimum economic system operation to occur, it is necessary to evaluate those transmission losses so that generation may be properly allocated among the various stations comprising the network. For example, it is obviously uneconom-ical to transmit equal amounts of power to a load from generating stations located at unequal distances from the load, assuming that both stations can produce power at the same cost. Therefore, it is necessary to reduce the power output of the distant station and increase the output of the closer station, so that the costs of power delivered to the load from the two stations are equal.

The calculation of transmission losses and penalty factors assigned to the various stations to correct for those losses is well known to those skilled in the art, and is described in an article entitled Evaluation of Methods of Coordinating Incremental Fuel Costs and Incremental Transmission Losses, by L. K. Kirchmayer and G. W. Stagg, AIEE Transactions, vol. 71, 1952, and in an article entitled Transmission Losses and Economic Loading of Power Systems, by L. K. Kirchmayer and G. H. McDaniel, published in General Electric Review, October, 1951.

The penalty factor for a generating station may be inserted by energizing the motor 32 as previously described, to turn the rotor of selsyn 31 a specific number of turns. This increases or decreases the generation of the station relative to that indicated by the system control signal, which is supplied to the stator of selsyn 31. In order to provide a visual indication of the penalty factor of the station at all times, the motor of selsyn 31 may be mechanically connected through suitable gearing to a pointer 34 operating in conjunction with a sale 35. The scale 35 may be calibrated in terms of penalty factor.

In general, the various stations of the power system will be at various points from the load center and, hence, will have different penalty factors. Therefore, it is generally necessary to provide a penalty factor unit 33 for each controlled station in the system. It is understood that although in the diagram of Fig. 1 only two penalty factor units are shown, the number of units in a system would correspond to the number of controlled generating stations which, of course, is not limited to any particular number.

The system control signal is supplied from the rotor of control transformer selsyn 31 in the penalty factor unit 33 to a frequency divider 36. The frequency divider 36 reduces the control signal frequency to make it more suitable for transmission. The control signal, whose frequency is now reduced, may then be transmitted by a carrier current transmitter 37 to the individual station affected. It is to be understood that the control signal may be sent to the individual stations by any conventional means such as leased wire, microwave transmissions, etc. For example, transmission means that are suitable for the present use are described in US. Patent 2,701,329 issued February 1, 1955 to E. E. Lynch and G. S. Lunge, and assigned to the same assignee as the present invention.

It is noted at this point that the entire system requires only one primary detector to detect frequency deviation and one primary detector to detect tie line load deviation, both being located at the central control station. 1

However, the central station will contain a penalty fac- 8 tor unit 33 and control signal transmission means for each controlled station in the network.

Referring now to the control signal responsive means located at each generating station, as shown in Fig. 2, the control signal sent to each station is received by suitable means such as a carrier current receiver 40, and its original frequency is restored by a frequency multiplier 41. Thus, the output of frequency multiplier 41 is once again a control signal having a frequency f (Af-t-kAw). Of course, the control signal may have been modified by the penalty factor unit 33 (Fig. 1), but it is assumed for clarity of explanation that it has not been modified. It is noted that Fig. 2 illustrates only one station, and the control elements for only two generators within that station are shown. It is understood, however, that the equipment shown will be duplicated in each controlled generating station of the network, and that each station may comprise any number of individual generators, each with the specific pieces of control equipment which serve them.

The control signal from frequency multiplier 41 is supplied to one input of a servo amplifier 42, which is similar to the servo amplifier 23 previously described, and may be identical, if desired. The second input to the servo amplifier 42 is from the system power line having a frequency f,,. Because of the similarity of the servo amplifiers 23 and 42, it is sufiicient here to note that the servo amplifier 42, compares the frequency of the control signal f +(Af+kAw) with the system frequency f,,, and produces rotation of an output shaft 43 at a speed proportional to the frequency or phase difference between the two signals and in a direction determined by the polarity of the difference.

The shaft 43 is connected to the input of a gear box 44, which has output shafts 45, 46, 47, 4S, and 50. The gear box 44 is of conventional design and it may be arranged to have the gear ratios between its input shaft and its output shafts of any desired values. In the present instance, it has been found satisfactory to have ratios between the input shaft and output shafts 45, 46, 47, 48, and 50 of 1:1, 4:3, 2:1, 4:1, and 10:1, respectively, so that output shaft 45 rotates at the same speed as the input shaft and the other output shafts rotate at fractions of this speed. The output shafts 45, 46, 47, 48, and 50 are connected to the rotors of differential selsyns 51, 52, 53, 54, and 55, respectively; and the three-phase stator windings of these selsyns are energized from the system power line having frequency f Therefore, the frequency of the voltage induced on the three-phase rotor winding of differential selsyn 51 will be f,+(Af+kAw), the frequency of the voltage induced on the rotor of selsyn 52 will be 1, plus of the area requirement signal (Af+kAw), the rotor of selsyn 53 will have an induced voltage of frequency f plus 50% of the area requirement signal, the induced voltage on the rotor of selsyn 54 will be of frequency 1 plus 25% of the area requirement signal, and the frequency of the induced voltage on the rotor of selsyn 55 will be f plus 10% of the area requirement signal.

The induced voltages on the rotors of the differential selsyns are each coupled to a contact of a station control rate selector switch 56, the rotors of selsyns 51, 52, 53, 54, and 55 being connected to contacts 57, 58, 60, 61, and 62, respectively, and the system power line having frequency f is connected to contact 63. It is pointed out that the rotors of the differential selsyns which are connected to selector switch 56 have threephase windings thereon; and therefore, selector switch 56 would normally have three banks of contacts but is shown as a single-bank switch only for purposes of explanation. A station control rate selector switch 56 is provided for each of the generators comprising the controlled station, so that the control rate may be individually selected foreach generator in accordance with its response capabilities. This is necessary because genertrolled generator.

1 ating equipments of various ages and physical conditions have different rates at which they are capable of varying their output. In the drawing, only two such control rate selector switches are shown connected in parallel; but it is understood that any number may be employed which corresponds with the number of generators under control within the station.

A movable contact selector arm 64 of each unit control rate selector switch 56 is connected to the stator winding of a control transformer selsyn 65, which is thus energized by one of the three-phase voltage outputs of the differential selsyns 5155. The control transformer selsyn 65 and a motor 66, to which the selsyn rotor is connected, constitute a load proportioning unit 67, which is somewhat similar in function to the penalty factor unit 33 previously described with reference to Fig. 1. Each load proportioning unit 67, one of which is required for each generator controlled, serves to allow the control signal supplied from the control rate selector switch 56 to be modified in accordance with the economic considerations for each of the generator units.

At the generating station, a voltage proportional to the incremental cost of generated power at the station is provided from a potentiometer 68 connected across a standard voltage source 70. The potentiometer 68 is similar in function to potentiometer 27 in the central control station, and has a movable contact pick-off arm which is connected to an output shaft 71 of the gear box 44.

The potentiometer 68 serves to integrate the output of the servo amplifier 42, and the voltage present on the potentiometer pick-off arm represents the incremental cost of power generated by that station. If no penalty factor has been applied to the control signal for that station (or if the penalty factor is unity), of course the voltage present on the pick-off arm of potentiometer 68 is the same as that present on the pick-off arm of potentiometer 27 in the central control station. A voltmeter 72 connected to indicate the voltage on the arm of potentiometer 68 may be calibrated in terms of station incremental cost.

The incremental cost voltage appearing on the movable contact arm of potentiometer 68 is supplied to parallel-connected potentiometers 73, one of which may be required for each generator within the station. Each potentiometer 73 serves to multiply the incremental cost voltage by a fuel cost factor, and produce on its movable contact pick-off arm a voltage which is proportional to the incremental heat rate for the corresponding generator. If the fuel cost is the same for all of the gen erators within the station, only one potentiometer need be used, and all of the outputs taken from the contact pick-off arm of the single potentiometer. However, it is possible that the fuel cost will vary from generator to generator, and the settings of the potentiometer pickolf arms will vary slightly from one to another to take this difference into account.

The incremental heat rate signal from the pick-off arm of each potentiometer 68 is supplied to the input of a function generator 74. Although the present invention is not limited to the use of any particular function generator, one which is known to be suitable will be described later with reference to the diagrams shown in Figs. 5-7.

Each function generator 74 serves to produce an output signal that is proportional to the power output of the generator at which occurs the incremental heat rate corresponding to the input signal. The output signal of each function generator 74 is supplied to an input of a balancing amplifier 75 which will be described hereafter. A second input of each balancing amplifier 75 receives a signal from a telemeter receiver or other conventional means, which is proportional to the output of the con- Each balancing amplifier then cornpares the amplitudes of the two input signals; and if there is a difference in their amplitudes, energizes the motor 66 in the load proportioning unit 67 to modify the control signal output of the control transformer selsyn 65 to cause the output of each generator to be varied until the signal proportional to that output is the same as the output signal of the corresponding function generator 74. Thus, each generator tends to adjust its output to the point of most economic loading.

If the generators are being run by turbines, it may be desirable to adjust the function generators 74 to produce an output signal that is proportional to the valve position of the turbine, rather than the power output of the generator. In this case, the signal connected to the second input of the balancing amplifier will be proportional to the valve position of the turbine, rather than to the actual power output of the generator.

The control signal output of the rotor of each control transformer selsyn 65 is supplied to one input of an electronic positioning unit 76. The electronic positioning unit 76 serves to compare the phases (or frequencies) of two input signals and, if they do not agree, to provide an output which will actuate other apparatus to correct the difference. The present invention is not limited to the use of any particular electronic positioning unit, but one such device which is known to be suitable is described in copending application Serial No. 395,118 filed November 30, 1953 by J. I. Larew, now Patent No. 2,796,556, issued June 18, 1957, and assigned to the same assignee as the present application; the schematic diagram of that device is shown in Fig. 9 of the present application and will be described hereafter.

As previously mentioned, one of the inputs to the electronic positioning unit 76 is from the rotor of control transformer selsyn 65, which signal has a frequency equal to f,,+(Af+kAw), if it is assumed for ease of explanation that the control signal has not been modified by the penalty factor unit or the load proportioning unit. The second input to the electronic positioning unit is from the rotor of a control transformer selsyn 77, whose three-phase stator winding is energized by the system line power having frequency f,,. From the previous description of the operaiton of a control transformer selsyn, it is apparent that the frequency of the voltage induced in the single-phase rotor winding of selsyn 77 will be the same as that which energizes the three-phase stator winding, unless the rotor is rotated to add to or subtract from that frequency. The electronic positioning unit 76 serves to compare the frequency of the signal connected thereto from the rotor of control transformer selsyn 65 in the load proportioning unit 67 with the frequency of the signal connected thereto from the rotor of control transformer selsyn 77. If the frequencies of the two signals differ, the electronic positioning unit 76 energizes a reversible synchronizing motor 78, which changes the speed-level setting of a governor of the generator prime mover (not shown). The synchronizing motor 78 is reversible, and the operation of the electronic positioning unit 76 is such that it can cause the synchronizing motor to turn in either direction, depending on the direction of frequency difference between the two input signals to the unit. The synchronizing motor 78, which is connected to the governor 80, is also connected through a reduction gear assembly 81 to the rotor of control transformer selsyn 77. Thus, as the synchronizing motor 78 rotates in response to an output from the electronic positioning unit 76, it also rotates the rotor of control transformer selsyn 77, and thus changes the frequency of the signal induced on the rotor. Therefore, the rotor of selsyn 77 will be so rotated that the frequencies of the signals supplied to the two inputs of the electronic positioning unit 76 agree. Thus, the speed-level setting of the governor 80 is changed by an amount proportional to the integral with respect to time of the area requirement signal (Af-l-kAw).

Consider now the operation of the tie line load-frequency control system shown in Figs. 1 and 2 in response to load changes in the local area, which is controlled by the system, and in the remote area to which the local area is joined by one or more tie lines. For purposes of explanation, it will be assumed that the change in load occurs at either the local area or the remote area, and the complex situation in which load changes occur simultaneously in both areas will not be considered. It is pointed out, however, that in the more complicated situation, the operation of the control system is basically the same as in the simple situation involving only one load change; and if the control system is installed in both the local and remote areas, the system will be quickly restored to normal, even in the case where the loads at the local and remote areas change simultaneously in varying proportions.

First, consider the case in which the load in the local area decreases, thus resulting in an excess of generation in this area. It is apparent that, in this case, the system frequency will increase and the power transmitted over the tie line from the local area to the remote area will also increase. Thus, the signal w from the telemeter receiver proportional to actual tie line load supplied to the tie line load controller 20 will be larger than the signal w which is produced within the tie line load controller and is proportional to the prescheduled value of tie line load. The output shaft 22 of the tie line load controller will rotate at a speed kAw proportional to the tie line load deviation Aw=w --w Therefore, because the actual tie line load is greater than the prescheduled value, output shaft 22 will rotate in a direction so that the signal supplied to the input of the servo amplifier 23 from the rotor of differential selsyn 21 will be equal to (f -HcAw) where Aw is negative.

The actual system frequency f is greater than the standard frequency f and, therefore, when the standard frequency f, is compared with the system frequency f by the servo amplifier 23, the shaft 25 will rotate at a speed proportional to -(Af+kAw), the two deviation signals adding together to produce the area requirement signal which is also proportional to change in incremental cost of generated power.

The area requirement signal having frequency is then combined with a signal having the system frequency f, by the selsyn 26. In this particular case, the shaft 25 turns the rotor of selsyn 26 in a direction to subtract the area requirement frequency from the system frequency in order to reduce the power generated within the controlled area. The frequency of the signal supplied from the rotor windings of selsyn 26 to the stator windings of the control transformer selsyns 31 in the penalty factor units 33 is equal to 12,- (Af-l-kAw).

As the shaft 25 rotates, it also positions the movable contact pick-off arm of potentiometer 27 to provide a voltage thereon which represents the incremental cost of generated power for the system load. This incremental cost is indicated to the operator by the voltmeter 30.

Assuming that the penalty factor for the station is not changed and the motor 32 in the penalty factor unit is not energized, the control signal passes through the penalty factor unit with its frequency unchanged and is then transmitted to each generating station through the frequency divider 36 and transmitter 37 or other conventional means.

At each individual generating station (Fig. 2), the control signal is returned to its proper frequency by the frequency multiplier 41 and is fed into the servo amplifier 42, which serves to subtract the system frequency f,, from the signal and cause the shaft 43 to rotate at a speed proportional to the area requirement signal (Af+kA-.r).

Various proportions of this signal are again combined with the system frequency signal f by the differential selsyns 51-55. The unit control rate selector switches 56 permit the selection of any one of variously proportioned control signals (or only the system frequency from contacts 63) to be sent to the control signal responsive means for the individual generators within the station.

For purposes of illustration, it is assumed that the 100% control signal is selected from the uni-t control rate selector switches 56 and supplied to the stator windings of control transformer selsyns 65 in the load proportioning units 67. As the shaft 43 rotates, it positions the movable contact pick-off arm of the potentiometer 68 from which is derived a voltage proportional to the incremental cost of delivered power for the station. This incremental cost voltage is modified by the fuel cost factor of the various units by potentiometers 73, and the resultant incremental heat rate voltage is fed to the function generators 74. The function generators 74 serve to produce an output voltage which is proportional to the output of each generator corresponding to the incremental heat rate voltage sent to the function generators, and the balancing amplifiers 75 serve to compare the actual outputs of each generator unit with the desired outputs as derived from the function generators. If these two factors are not the same, the balancing amplifiers 75 produce voltages to cause rotation of the motors 66 in the load proportioning units to add to or subtract from the frequency of the station control signal to attain the desired output from the individual generators. A generator control signal is supplied from the rotor of each control transformer selsyn 65 to one of the inputs of each electronic positioning unit 76. This signal, which has a frequency f,, (Af+kAw), assuming that no economic factor modification has occurred, is compared in phase or frequency with the system frequency f and, because there is a difference in phase or frequency between the signals, synchronizing motor 78 is energized to re-set downwardly the speed-level setting of governor 8t). Simultaneously, the rotor of control transformer selsyn 77 is rotated to send back to the electronic positioning unit a signal whose frequency is the same as that provided the other input of the electronic positioning unit from control transformer selsyn 65. Thus, the motor 78 rotates until the control signal frequency f Af+kAw) equals the system frequency f and the speed-level setting of governor 80 remains at its new level.

It is again pointed out that this same action occurs for each of the controlled generators within each station; and, because each generator unit is provided with a unit control rate selector switch 56 and with an economic control portion comprising a function generator 74 and a balancing amplifier 75, the speed-level settings of the governors for the various generators may differ one from the other. Similarly, because in the central control station there is a penalty factor unit 33 for each controlled station of the network, the control signals received by the stations may differ from each other.

As a further example of the operation of the system, consider the case in which the load in the local area in creases, thus causing a decrease in the frequency of the system and a decrease in tie line load from the local area to the remote area. In this case, the operation of the various components of the system would be like that previously described for the case where the load in the local area decreased, except that the direction of rotation of the various motors and selsyns would be opposite to that previously described. For example, the frequency deviation A and the tie line load deviation Aw would both be positive. Thus, the rotation of the shaft 25 in the system master controller would be in an opposite direction from that previously described; and the frequency of the control signal derived from the selsyn 26 in the master controller would be greater than the frequency 'f of the system. At each individual controlled station the "ascends electronic positioning units 76 would cause the synchronizing motors 78 of the generators to turn in a di rection so as to increase the speed-level settings and increase the generation, rather than decreasing it as was the case in the first example considered.

A different situation arises when the load in the remote area changes. For example, if the load in the remote area decreases, the system frequency f tends to increase and the power flow over the tie line from the local area to the remote area decreases. Therefore, the frequency f is greater than the standard frequency f and the frequency deviation Af is negative. However, the actual tie line load w decreases, thus becoming less than the scheduled load W and the load deviation Aw is positive. When the servo amplifier effectively takes the difference between the frequencies of its two input signals, kAw and M tend to cancel each other; and the shaft 25' will not rotate. Therefore, the signal sent from the control station to each individual generating station will be a signal having frequency f in the absence of a change of penalty factor. At the individual stations, this frequency is compared with the system frequency f and, because these two frequencies are the same, there will be no control action exercised in the individual stations. Therefore, the outputs of the generators in the local area will be unchanged by such a load change in the remote area.

As a final example, consider the situation when the load in the remote area increases, thus tending to decrease the system frequency, and increase the power flow from the local to the remote area. In this case, the frequency deviation M is positive, and the tie line load deviation kAw is negative. Thus, it is apparent that once again the shaft 25 will not rotate, and the control signal received by each of the stations will once again have the system frequency f which will initiate no change in the speed-level setting of the governors of the generators.

The consideration of the last two examples brings out another of the outstanding advantages of the invention. It was noted that although the control signal was such as to cause no change in the speed level settings of the governor generators, nevertheless the control signal was present. Thus, the control signal may be continuously monitored to insure that it is present, even though the corrective portion of the control signal may be zero.

If the control system is to be used to control the generation of a local power system, which is operating independently and is not connected by a tie line to a remote power system, the tie line load controller 20 and the differential selsyn 21 may be dispensed with or made inoperative and the system power line having frequency f connected directly to the second input of the servo amplifier 23. In that case, any load changes in the independent local area will cause a change in system frequency, which will cause the power system to vary its generation to return the system frequency to its standard value, and thus cause the generation output to match the system load.

In some interconnected power systems, it is customary for one system to vary its generation to maintain only the tie line load at its prescheduled value and to ignore system frequency changes. In that case, the servo ampliher 23, frequency standard 24 and differential selsyn 21 may be eliminated or made inoperative, and the output shaft 22 of the tie line load controller 20 connected directly to the rotor of the selsyn 26 and the potentiometer 27. The frequency of the control signal will be dependent only on the tie line load deviation, and the generation of the local system adjusted only to maintain the tie line load at its prescheduled value without regard for system frequency.

Another advantage of the present invention is that various units within a station, or even stations themselves, may be taken out of or added to the control system, without affecting its operation. If, for example, a station is taken out of operation for some reason, its share of the output will be automatically reapportioned among the remaining stations. Thus, the control systemwill continue to operate and will continuously adjust the output of the stations and generators to account for load changes within the local area, in order to retain the tie line load at its prescheduiled value and the frequency of the system at its desired point.

Also, it is apparent that control of the individual generator units is independent of the characteristics of the synchronizing motors. This is so because there is feedback from each synchronizing motor 78 through the control transformer 77 to its electronic positioning unit 76, so that each motor is individually controlled.

It is pointed out that the economic factor control equipment, which is one of the most important features of the present invention, permits continuous, automatic adjustment of the outputs of each generating station and generators to maintain the most economic operation possible.

It will be recalled that the tie line load deviation signal Aw is provided by the tie line load controller 20 in the central control station (Fig. l). The tie line load controller 20 receives a direct current signal w proportional to actual tie line load, compares it to a signal w provided within itself and proportional to desired or scheduled tie line load, and causes rotation of the output shaft 22 at a speed proportional to the difference Aw. The present invention is not limited to the use of any particular device for the tie line load controller. However, apparatus that operates satisfactorily in this application is described in copending application Serial No. 395,117 filed November 39, 1953 by I. J. Larew and K. N. Burnett, now Patent No. 2,753,505, issued July 3, 1956 and assigned to the assignee of the present invention. Reference is made to that application for a detailed description of the device, which will now be described in more general terms with reference to its schematic circuit diagram shown in Fig. 3.

The direct current input signals w proportional to tie line load, which is to be compared with a reference signal w proportional to desired or scheduled tie line load, may be connected between a pair of input terminals and 91. A smoothing filter comprising a resistor 92 and a capacitor 93 serves to shunt any alternating current ripple voltage that might be superimposed on the direct current signal. The reference direct current signal w is derived from a potentiometer 94 connected across a pair of standard direct voltage cells 95a, 95b. The movable contact arm of the potentiometer 94 is connected in series circuit relationship with the capacitor 93. A voltmeter 96 is connected between the movable arm of potentiometer 94 and the juncture of the cells 95a, 95b; the voltmeter may be calibrated directly in terms of desired tie line loa-f W and the load schedule varied by changing the position of the potentiometer movable arm.

The series circuit formed by capacitor 93 and p0tentiometer 94 is coupled to a means for converting direct current to alternating current. Such means comprises a solenoidoper-ated chopping device 97 having a movable armature 98 and a pair of fixed contacts 100 and 101 connected to opposite ends of the center tapped primary winding of a transformer 102. The center tap of the transformer primary Winding is connected to the movable arm of a potentiometer 103, one endof which is connected between the cells 95a, 95b through a potentiometer 104 adapted to function as a variable resistance. The function of potentiometers 103, 104 will be explained hereafter.

The operation of a device such as the chopper 97 is well known in the art, and will not be described in detail. Briefly, however, the chopper is actuated by an operating winding 105 that is energized from a low voltage'sec' ondary winding 106 of a main power transformer 107,

'15 whose'primary winding 108 may be connected to the 60-cycle power line of the system. As the operating winding 105 of chopper 97 is energized by the 60-cycle alternating current flowing therethrough, the armature 98 is caused to vibrate between contacts 100 and 101 at the frequency of the current through operating winding 105. Thus, during one-half of each cycle of the energizing alternating current supplied to the operating winding, current flows through one-half of the primary winding of transformer 102; and, during the other half cycle of the energizing current, current flows through the other half of the primary winding. The direction of this current flow depends on whether the direct current signal (w w supplied by the series circuit comprised by capacitor 93 and potentiometer 94 is positive or negative; that is, the current flow through the primary winding of transformer 102 may be from the ends of the winding to the center tap if the deviation signal Aw is of one polarity, and from the center tap to the ends of the winding if the signal is of the other polarity. An alternating current signal is induced in the secondary winding of transformer 102 which is in phase with the voltage appearing across operating winding 105 if the direct current signal Aw supplied to armature 98 is of one polarity, and which is 180 out of phase with the voltage across operating winding 105 if the direct current signal Aw is of the opposite polarity. Because the phase of the voltage across operating winding 105 (which is derived from secondary winding 106 of transformer 107) is taken to be in phase with the 60-cyc1e line voltage, the 60-cycle line voltage which energizes the primary winding 108 of transformer 107 may be used as an alternating current reference voltage. Hence, by comparing the phase of the alternating current signal induced across the secondary winding of transformer 102 with the phase of the 60-cycle line voltage supplied through primary winding 108 of transformer 107, an indication may be obtained of the polarity of the direct current signal Aw supplied by capacitor 93 and potentiometer 94.

The alternating current signal induced in the secondary winding of transformer 102 is coupled to the control grid of the first stage of a conventional three-stage resistance-capacitance coupled amplifier, comprising triode electron discharge devices 110, 111, and 112, along with the necessary anode and cathode resistors and the interstage resistor-capacitor coupling circuits. Voltage is supplied to the anode of the discharge devices 110, 111, and 112 by a half-wave rectifier 113. Rectifier 113 is energized from a secondary winding 114 on main power transformer 107; and the direct current output thereof is supplied through a conventional resistance-capacitance filter network 115 to the respective anode electrodes of electron discharge devices 110, 111, and 112.

The amplified alternating current signal appearing on the anode of triode electron discharge device 112, which is of opposite phase from that appearing across the secondary winding of transformer 102, is supplied by means of a coupling capacitor 116 and grid biasing resistor 117 to the control grids of a pair of duo-triode electron discharge devices 118 and 120. The anodes of duo-triode 118 are connected together and to one end of a secondary winding 121 of power transformer 107, and the anodes of duo-triode 120 are connected together and to the other end of secondary winding 121. The four cathodes of the duo-triode tubes 1118 and 120 are interconnected and have a common cathode resistor 122.

From the foregoing description, it is apparent that the anodes of duo-triode 118 are energized by a potential that is 180 out of phase with the potential that energizes the anodes of duo-triode 120. A center tap on seconary winding 121 of transformer 107 is connected to a field winding 123 of a two-phase motor 124, whose other winding 125 is connected through a capacitor 126 to the 60-cycle line voltage input, previously mentioned as being the phase reference alternating current voltage for the device. The capacitor 126 in series with winding 125 serves to insure that the current through that winding is out of phase with the current through winding 123 for reasons which will be pointed out later. A capacitor 127 connected across winding 123 serves as a filter and to tune the output circuit of the final stage.

As was previously mentioned, an alternating current signal is provided on the input of the three-stage amplifier which is in phase, or 180 out of phase, with the 60- cycle line voltage input. Therefore, the alternating current signal supplied to control grids of duo-triodes 118 and is also in phase, or 180 out of phase, with the 60-cycle line voltage which energizes primary winding 108 of transformer 107. Hence, the alternating current signal is either in phase with the anode voltage of duotriode 118 and 180 out of phase with the anode voltage of duo-triode 120, or vice versa. If there is an alternating current signal on the grids of the duo-triodes 118 and 120, one of the duo-triodes will conduct more heavily than the other; and there will be more current through motor winding 123 during one half-cycle than during the other half-cycle to cause the motor 124 to rotate in one direction or the other. Of course, the direction in which the motor turns is determined by the polarity of the direct current deviation signal Aw, and its speed of rotation is related to the amplitude of the deviation signal. It is pointed out that at balance, when there is no deviation signal Aw, equal currents flow through motor winding 123 during both halves of each cycle, and the motor 124 does not rotate.

The motor 124 is connected to a feedback signal developing means comprising a direct current tachometer 128 which when rotated in one direction produces a direct current feedback voltage of a certain polarity, and when rotated in the opposite direction produces a direct current feedback voltage of the opposite polarity, the amplitude of the voltage being proportional to the speed of rotation. The direct current voltage produced by tachometer 128 as motor 124 rotates is fed back to the deviation signal producing circuit, previously described, to linearize the output speed of motor 124- with respect to the direct current deviation signal input. One side of the output circuit of tachometer 128 is connected to the juncture of cells 95a, 95b, and the other side is connected to the movable contact of a three position selector switch 130. One of the fixed contacts of the three position selector switch is connected directly through a resistorcapacitor network 131 to the previously unconnected end of potentiometer 103, whose other end is connected to potentiometer 104, the two potentiometers functioning as a voltage divider. The remaining two fixed contacts of selector switch 130 are connected to the network 131 through a pair of batteries 132 and 133, arranged with opposite polarities. By proper selection of the fixed contacts of selector switch 130, a positive or negative voltage may be added in series with the direct current feedback voltage developed by tachometer 128. Because the movable contact of potentiometer 103 is connected to the center of the primary winding of transformer 102, the feedback signal is added to the direct current deviation signal developed by capacitor 93 and potentiometer 94.

It is apparent that as various amounts of the direct current feedback signal from tachometer 123 are fed back to the deviation signal developing circuit comprising potentiometer 94 and capacitor 93, the slope of the curve of the output motor speed versus input volts will vary, and will be stabilized thereby. It is also apparent that if the direct current feedback voltage is too small, this output curve will cease to be linear. It is the function of the voltage divider comprising potentiometers 103 and 104 to set the amount of direct current feedback voltage available. Thus, the setting of the movable contact of potentiometer 104 is determined by the amount of DC.

feedback voltage required to linearize the output curve of motor speed versus deviation signal Aw, and is influenced by the slope desired for this curve. The desired slope for the curve of output speed versus input deviation signal may be obtained by adjustment of the movable contact arm of potentiometer 103.

For purposes of explanation and description, let it be assumed that during the first half of each cycle of the GO-cycle reference voltage, current flows through the upper half (as seen in the drawing) of the primary winding of transformer 102, and during the second half of each cycle, current flows through the lower half of the winding. Let it also be assumed that the direct current tie line load signal w connected between input terminals 90 and 91 is more positive than the reference signal w taken from potentiometer 94 and, therefore, that current will flow downwardly through the upper half of the primary winding of transformer 102 during the first half of each cycle, and upwardly through the lower half of the winding during the second half of each cycle. Thus, it is assumed that the alternating current signal appearing across the secondary winding of transformer 102 is in phase with the 60-cycle voltage supplied across the primary winding 108 of power transformer 107. Therefore, because the amplifier comprising discharge devices 110, 111, and 112 has an odd number of stages, the signal appearing on the anode of d scharge device 112 is 180 out of phase with the reference 60-cyce supply voltage. Now, let it also be assumed that the voltage on the anodes of duo-triode 118 is in phase with the 60-cycle power line reference voltage, and thus voltage on the anodes of duo-triode 120 is 180 out of phase with this reference voltage. Therefore, if during the first half of each cycle the alternating current signal appearing on the control grids of the duo-triodes is negative, while the voltage appearing on the anodes of duo-triodes 118 is positive and the voltage on the anodes of duo-triode 120 is negative, no current will flow through either of the duo-triodes if the alternating current signal is great enough. However, during the second half of each cycle, the alternating current signal placed on the control grids is positive, the anodes of duo-triode 118 are negative, but anodes of duo-triode 120 are positive. Therefore, current will fiow through duo-triode 120 during the second half of each cycle. Thus. depending on the connections, the phase of the current flowing through motor winding 123 is either 90 ahead or 90 behind the phase of the current flowing through motor winding 125, and the motor will turn in a certain direction. As the motor 124 turns, tachometer 128 generates a. direct current feedback voltage, part of which appears across potentiometers 103 and 104. That feedback voltage is of proper polarity to act as a negative feedback, in the manner well known in electronic circuitry. Thus, the direct current deviation signal tends to be reduced as the motor speed is increased and the net effect is to cause the speed of the motor 124 to stabi ize at a speed kAw which is proportional to the direct current tie line load deviation signal Aw.

Conversely, if the direct current tie line load signal w connected across terminals 90 and 91 is negative with respect to the reference voltage set on potentiometer 94, during the first half of each cycle current flows upwardly through the up er half of the primary Winding of transformer 102. and flows downwardl during the lower half of the winding during the second half of each cycle. Thus, the signal appearing across the secondary winding of the transformer is 180 out of phase with 60-cvcle line voltage which ener izes primary Winding 108 of the power transformer 107. Again, the alternating current signal appearing on the anode of discharge device 112 is 180 out of phase with the input, and thus is in phase with the 60-cycle reference voltage. During the first half of each cycle, as was pre iously pointed out, the anodes of duo-triode 118 are positive, and the anodes of duo-triode are negative. Thus, during thefirst half of each cycle, in the present illustration where the direct current deviation signal is negative rather than positive, the control grids of duo-triode 118 are positive at the same time that the anodes thereof are positive. Therefore, current flows through duo-triode 118 during the first half of each cycle. Duringthe second half of each cycle, the signal appearing on control grids is negative; and, therefore, if the signal is of sufiicient magnitude, neither of the duo-triodes in the final stage conduct. Thus, it is seen from the two examples just described, that when the direct current deviation signal Aw is positive, current flows through motor winding 123 during the second half of each cycle, and when the direct current deviation signal is negative current flows through the motor winding during the first half of each cycle. Because of the capacitor 126 in series with motor winding 125, the current flowing through motor winding 123 is either displaced in phase 90 ahead or 90 lagging the current through winding and the motor turns in opposite directions when the direct current deviation signal is positive or negative. A suitable direct current feedback signal will then be developed and fed back to the input of the mechanism in the manner described above.

It is noted that the mechanical connection between the motor 124 and the tachometer 128 may be extended to provide the connection 22 between the tie line load controller 20 and the selsyn 21, as shown in Fig. 1.

Figs. 4 and 4a illustrate a possible form of the servo amplifiers 23 and 42 previously mentioned with reference to Figs. 1 and 2, although the present invention is not limited to the use of this particular device. The servo amplifier illustrated forms the subject matter of copending application Serial No. 395,119 as previously noted.

It will be recalled that-the servo amplifiers 23 and 42 serve to compare the frequencies or phases of two electrical signals and produce rotation of an output shaft at a speed proportional to the frequency difference.

Referring to Fig. 4, it is seen that the servo amplifier comprises a discriminator 135, which will be later described in detail; a convention two-phase motor 136; and a conventional control transformer selsyn 137. One

' input to the discriminator may be from a frequency standard (as in Fig. 1) or other source (not shown), and the second input is. from the rotor of the selsyn 137. The stator winding of the selsyn 137 is energized by the signal whose frequency or phase is to be compared to that of the signal from the frequency standard or; other source, which is connected to the first input of the discriminator. If there is a difference in frequency or phase between the input signals to the servo amplifier, there is an output from the discriminator 135 that causes the motor 136 to rotate. The rotor of the selsyn 137 is connected to the output shaft of the motor and rotates with it, as does a. shaft 138 that is the output shaft of the servo amplifier.

As is well known, the voltage induced in the rotor winding of a control transformer selsyn has the same frequency as the voltage which energizes the stator winding of the transformer, if the rotor is not rotating. However, if the rotor is turning, the voltage induced across the rotor winding will differ from the stator voltage by an amount determined by the speed of rotation of the rotor. For example, if the stator is energized by a voltage having a frequency of 60 cycles per second, and the rotor is rotated at five revolutions per second, the voltage induced in the rotor winding will have a frequency of either 55 or 65 cycles per second, depending on the direction of rotation of the rotor. Thus, in the present instance, as the motor 136 turns in response to a phase or frequency difference between the servo amplifier input signals, it will vary the frequency or phase of the second input signal to the discriminator, which is taken from the rotor of the control transformer selsyn 137. It is apparent that, when the elements are connected as a servo amplifier in the manner shown in Fig. 4, the motor 136 will be caused to rotate the rotor of selsyn 137 and the output shaft 138 at a speed which is proportional to the phase or frequency difference between the first input signal to the discriminator 135 and the signal which energizes the stator winding of the selsyn 137.

Fig. 4a illustrates a possible form of the discriminator 135 that is embodied in the servo amplifier shown in Fig. 4. Referring now to Fig. 411, one of the two input signals, which must be approximately a sine wave and must not be grounded, may be connected to input terminals 140 and 141, and the second input signal connected to terminals 142 and 143 if the signal is not grounded, or to input terminals 142 and 144 if the signal is grounded. The input signal connected to terminals 140 and 141, which is taken to be the reference signal and will be hereafter referred to as signal A, produces a voltage drop across resistors 145, 146, and 147, connected in series between the input terminals resistors 146 and 147 are of the same value, so that the signals appearing across them are equal, and resistor 145 serves merely as a series dropping resistor.

The second input signal is assumed, for purposes of explanation, to be ungrounded and connected to termi- Hills 142 and 143. This signal, which will be hereinafter referred to as signal B, appears across a resistor 148 connected between terminal 143 and the junction of resistors 146 and 147. An inductance 150 is connected between input terminals 142 and the junction of resistors 146 and 147, and operates as a filter in conjunction with a capacitor 151 connected across resistor 148, which permits the application of an alternating voltage to terminals 152 and 143 that may be of other than a strict sine wave s ape.

The junction of resistors 145 and 146 is connected to the cathode of an electron discharge device 152 of the diode type, and input terminal 141 is connected to the cathode of a similar discharge device 153. It is now seen that at any instant, the signal appearing at the cathode of diode 152 is the vector sum of the signals appearing across resistors 146 and 148, and the signal appearing at the cathode of diode 153 is the vector sum of the signals appearing across resistors 147 and 148.

The anodes of diodes 152 and 153 are connected together through resistors 154 and 155, the juncture of which is connected to input terminal 143. Resistors 154 and 155 are of equal value so that, when diodes 152 and 153 are conducting equally, equal signals appear across the two resistors. Capacitors 156 and 157 are connected across resistors 154 and 155, respectively, and act in conjunction with the resistors to filter the output signal of the diodes. If the input signals to the circuit are such that equal signals appear on the cathodes of the diodes, the diodes conduct equally, and equal negative signals will appear on the anodes thereof. If the input signals are so related that unequal signals appear on the cathodes of the diodes, as will be later explained in detail, one diode will conduct more heavily that the other; and the D.C. voltage appearing at its anode will be more negative than that appearing at the anode of the other diode.

The D.C. voltages appearing across resistors 154 and 155 also appear across resistors 158 and 160, which serve as grid resistors for two D.C. amplifiers comprising triode electron discharge devices 161 and 162. The cathodes of the triodes 161 and 162 are grounded and the anodes of the triodes 161, 162 are connected through resistors 163 and 164, respectively, to a source of D.C. voltage, which will be later described. The control grid of triode 161 receives the negative D.C. signal appearing at the anode of diode 152, and the control grid of triode 162 receives the negative D.C. signal appearing at the anode of diode 153. A capacitor 165 is connected between the juncture of resistors 154 and 155 and the juncture of resistors 158 and and serves to permit the grid-to-cathode voltages of triodes 161 and 162 to be zero at the balance condition. That is, capacitor causes only the difference in the voltages appearingacross resistors 154 and 155 to be applied to the D.C. amplifiers, and maintains an alternating current ground at the juncture of resistors 154 and 155.

The output of the D.C. amplifiers is taken from the anodes of triodes 161 and 162 and is coupled through anti-hunt networks to the input of a power output stage. The signal appearing at the anode of triode 161 is connected through an anti-hunt network 166 and through a current limiting resistor 167 to the control grids of a pair of triode electron discharge devices 168 and 170, which are connected in parallel and have a grid resistor 171 across which the signal appears. Similarly, the signal appearing at the anode of triode 162 is connected through an anti-hunt network 172 and through a current limiting resistor 173 to the parallel-connected control grids of triode electron discharge devices 174 and 175, where the signal appears across a grid resistor 176. The cathodes of triodes 168, 170, 174, and 175 are connected through a biasing resistor 177 to the juncture of grid resistors 171 and 176. A capacitor 180 is connected between the juncture of resistors 171 and 176 and ground, and functions in a manner similar to that of the capacitor 165 previously described.

The parallel-connected anodes of triodes 168 and are connected to one end of a secondary winding 181 of a transformer 182, and the parallel-connected anodes of triodes 174 and are connected to the other end of the secondary winding 181. Thus, the voltage supplied to the anodes of triodes 168, 170 is out of phase with the voltage supplied to the anodes of triodes 174, 175, and the triodes cannot all conduct simultaneously. The primary winding 183 of transformer 182 is connected across power input terminals 184 and 185, to which 1l0-volt 60-cycle alternating current may be connected from the system lines. A filament winding 136 on transformer 182 provides voltages of the proper mag nitude for a filament 192 of triodes 168 and 174, and for a filament 193 of triodes 170 and 175.

One phase of a two-phase motor (such as the motor 136 shown in Fig. 4), which it is desired to have respond to a phase difference between the input signals, may be connected across output terminals 188 and 190 in the anode-cathode circuit of the output triodes. Terminal 188 is connected to the cathodes of triodes 168, 170, 174, and 175 through biasing resistor 177, and terminal 190 is connectedto a center tap of the secondary winding 181 of transformer 182. A jack 191 may be connected across terminals 188 and 190 so that the output may be monitored, if desired.

The anode voltage for the D.C. amplifier triodes 161 and 162 is provided by a power supply comprising a power transformer 194 and an electron discharge device 195 of the double diode type. The primary winding of the trans former 194 is connected to power input terminals 184 and 185, and secondary windings are connected to the double diode 195 in the conventional manner to provide full wave rectification. The D.C. output of the double diode 195 is filtered by a conventional filter 196 before being applied to the anodes of triodes 161 and 162. A resistor 197 is connected as a bleeder, which permits the removal of the charge from the filter capacitors when power is removed from the circuit. A filament 198 for diodes 152, 153 and a filament 199 for triodes 161 and 162 are supplied by another secondary winding of'transformer 194 in the usual manner.

For purposes of explanation, the operation of the amplifier circuit will be considered first with reference to two input voltages which are of sine wave shapein phase agreement and of such amplitude that equal signals appear across resistors 146, 147, and 148. In this case, the vector sum of the signals appearing across resistors 146 '21 and 148 appears on the cathode of diode 152, and the vector sum of the signals appearing across resistors 147 and 148 appears on the cathode of diode 153. Therefore, the signal appearing at the cathode of diode 152 is a sine wave, whose phase agrees with the phase of the two input signals and whose amplitude is twice that of the signal appearing across resistor 146 or resistor 148. The signal appearing on the cathode of diode 153 is zero, because it is the sum of equal negative and positive signals which cancel each other. During the first half of each cycle, the cathode of diode 152 is positive with respect to its anode, and diode 152 does not conduct; during the second half of the cycle, the cathode is negative with respect to the anode, and diode 152 does conduct. This causes a voltage drop across resistor 154 and charges capacitor 156. At the same time, because there is no signal appearing at the cathode of diode 153, no current flows through resistor 155 and no charge is built up across capacitor 157. Therefore, current flows through resistors 158 and 168 in the control grid circuits of triodes 1.61 and 162, and the control grid of triode 162 becomes positive with respect to its cathode while the control grid of triode 161 becomes negative with respect to its cathode. Thus, triodes 161 and 162 operate in push-pull fashion to cause a signal to appear across resistors 171 and 176 in the input circuit to the power stage. Therefore, the control grids of triodes 168 and 178 tend to go positive with respect to their cathodes, while the control grids of triodes 174 and 175 tend to go negative with respect to their cathodes. In this case, it is assumed that the signals appearing across resistors 1'71 and 176 are of such amplitude as to cause maximum current flow through triodes 168 and 170, while causing triodes 174 and 175 to cut off.

The anodes of triodes 168 and 170 are connected to one end of the secondary winding 181 of transformer 182, and the anodes of triodes 173 and 175 are connected to the other end of the secondary winding 181. Therefore, it is apparent that when the anodes of triodes 168 and 170 are positive, the anodes of triodes 174 and 175 are negative, and vice versa. Thus, if the control grids of triodes 168 and 178 are at the same potential as the control grids of triodes 174 and 175, the current flow through triodes 168 and 170' during one half of each cycle of the AC. input voltage is the same as that through triodes 174 and 175 during the other half of each cycle. However, in the present example, a positive signal appears on the control grids of triodes 168 and 170, while a negative signal appears on the control grids of triodes 174 and 175. Therefore, the current flow through triodes 168 and 178 exceeds that through triodes 174 and 175. This means that if one phase of the output motor 136 (not shown in Fig. 4a) is connected between terminals 188 and 190 in the cathode circuit of the output stages there is through that phase of the motor a pulse of current that corresponds in time to that portion of each AC. input cycle when the anodes of triodes 168 and 170 are positive. During the remaining half of each cycle, when the anodes of triodes 174 and 175 are positive and the anodes of triodes 168 and 170 are negative, there will be no current flow because no positive signal appears on the control grids of triodes 174 and 175. it is noted that the other phase of the motor should be energized by 60-cycle alternating current of the same phase as that which energizes the primary winding 183 of transformer 182, supplied to the motor through a capacitor to provide the usual 90 phase shift, as is well known to those skilled in the art. Therefore, when the input signals are in phase current flows through one phase of the two-phase motor during only half of each cycle, and the motor turns in a certain direction.

Next consider the case where the input signals dilfer in phase by 30 degrees. In this case, the signal appearing at the cathode of diode 152, which is the vector sum of the signals appearing across resistors 146 and 148, is approximately twice as large as, and 90 degrees out'of phase with, the signal appearing at the cathode of diode-153', which is the vector sum of the signals appearing across resistors 147 and 148. Thus, the control grid of triode 161 again tends to become negative and the control grid of triode 162 positive with respect to their common cathode connection, although not by as great an amount as in the first example. The control grids of triodes 168 and 170 again tend to go positive, and there is maximum current flow through these triodes during the positive portion of their anode voltage cycle. The control grids of triodes 1'74 and 175 tend to go negative but not by an amount sufficient to completely out off the triodes. Therefore, there is current How in the cathode circuit of the output power stage during both halves of each anode voltage cycle, but the current flow during one half of the cycle is approximately twice as great as that during the other hah of the cycle. Thus, the output motor turns in a direction determined by the phase of the greater signal with reference to the 60-cycle current which continuously energizes one phase of the motor. In another case, when the input signals are degrees displaced in phase, the signals appearing on the cathodes of diodes 152 and 153 are of equal amplitude; and the control grids of triodes 161 and 162 are at the same potential as their cathodes. Therefore, no signal appears on the control grids of 0utput triodes 168, 170, 174, and 175, and these triodes all have maximum current flow therethrough. In this case, equal currents flow in the cathode circuit of the output stage during both halves of each anode voltage cycle; the output motor is equally energized during both halves of each cycle and, consequently, does not turn. Thus, it is apparentthat a 90 phase displacement between the input signals to the amplifier is the reference displacement; and if the phase difference is less than 90, the motor will rotate in one direction at a speed determined by the phase difference.

It is apparent from the above explanation that, when the phase difference between the input signals to the-amplifier is greater than 90, the output motor should rotate in the reverse direction. This does occur because the flow of current through the diodes 152, 153 is such as to cause the control grid of triode 161 to tend to go positive and the control grid of triode 162 to go negative. Thus, the control grids of triodes 174 and 175 tend to go positive, while the control grids of triodes 168 and 170 tend to go negative so that maximum conduction occurs through triodes 174 and 175 and reduced conduction occurs through triodes 168 and 170. Therefore, the greatest current flow through the winding of the motor connected between terminals 188, occurs during the.

second half of each cycle, rather than during the first half, as when the phase diifcrence was less than 90. Therefore, the motor rotates in the opposite direction. It is noted that under conditions in which the amplitude of the input signals appearing across resistors 146, 147, and 148 are not equal, the only effect on the operation of the circuit is to reduce its sensitivity. This effect occurs because the sum and difference signals appearing at the cathodes of diodes 152 and 153 are not in quadrature, but are at a smaller angle; and consequently, motor torque is somewhat reduced.

It will be recalled that in connection with Fig. 2 it was stated that a voltage proportional to the incremental cost of generated power at a controlled station is provided from a potentiometer 68 connected across a standard voltage source 70. The voltage present on the potentiometer pick-off arm represents the incremental cost of power generated at that station. That incremental cost voltage is then multiplied by a fuel cost factor by the potentiometers 73 to provide voltages proportional to incremental heat rate, which are supplied to the function generators 74. Each function generator 74 serves to provide a direct voltage whose amplitude is proportional to the power output of a corresponding generator at which occurs the incremental heat rate corresponding to the input signal from the potentiometer 73.

Function generators of the type required for this application are well known in the art and readily available commercially. Examples of suitable devices may be found in the book Electronic Analog Computors, by Korn and Korn, McGraw-Hill Book Company, Inc., New York, New York, 1952. Although the present invention is not limited to the use of any particular function generator, apparatus that has been found to operate satisfactorily will be described hereafter with reference to Figs. -7.

In efiect, the function generator shown in Fig. 5 is a voltage divider comprising two resistances, one being fixed and the other variable in discrete steps, a new resistance step being introduced each time a predetermined input voltage is attained. The input signal may be supplied between a pair of terminals 200 and 201, terminal 200 being grounded; and fixed resistors 202 and 203 are connected in series between input terminal 201 and ground. The resistor 203 has a plurality of resistance arms 204, 205, and 206 connected in parallel therewith, which contain rectifier elements 204a, 205a, and 206a, respectively; variable resistances 204b, 205b, 2061:, respectively; and variable direct voltage sources 204a, 205e, 2060, respectively, with a rectifier element, a variable resistance, and a variable voltage source connected in series in each arm. If the direct voltage signal supplied between the input terminals 200 and 201 is positive with respect to ground, the connections of the rectifier elements 204a, 205a, 206a and the variable voltage sources 2040, 205e, 2060 are such as to oppose conduction through each of the resistance arms 204, 205, 206 until the voltage appearing across the resistor 203 is greater than the voltage of a source 2040, 205e, or 206C. Of course, if the input signal is negative, the polarities of the rectifier elements and the variable direct voltage sources would be reversed.

Looking only at that portion of the circuit thus far described, as the input voltage at the terminals 200, 201 increases from zero, the voltage at a point 207 between the resistors 202 and 203 increases proportionally until it is equal to one of the voltages provided by the variable voltage sources 2040, 205a, and 206C. Any increase of the voltage at point 207 above any one of the voltages set in by the variable voltage sources 2040, 205e, 2060 causes a corresponding rectifier element 204a, 205a, 206a to conduct and changes the basic voltage divider ratio to another value; that is, when one or more of the rectifier elements 204a, 205a, 206a conducts, one or more of the variable resistances 204b, 205b, 2061) is put in parallel with the fixed resistance 203. If it is assumed that the voltages set in by the variable voltage sources 2040, 205c, 206c are all diiferent and greater than zero, a plot of the voltage at point 207 versus the input voltage would take the general form of the curve shown in Fig. 6. The intercept points 208, 210, and 211, of course, are determined by the voltages set in at the variable voltage sources 204e, 205e, 206a, and the slopes of the curves are determined by the value of resistors 202 and 203 and by the settings of the variable resistances 204b, 2051;, 20617, as will be apparent to one skilled in the art.

Because the turbine whose speed level setting is controlled by the function generator has certain fixed limits of control, various limit controls are provided in the function generator. A high-limit control is provided by an arm 212 similar in function to the intercept control arms 204, 205, and 206, previously described. The highlimit control arm 212 comprises a series-connected rectifier element 213 and a source of variable direct voltage 214 connected in parallel with the arms 204, 205, 206 through a rectifier element 215. Neglecting for the mo ment the rectifier element 215, when the voltage at point 207 becomes greater than that set in by the variable voltage source 214, the rectifier element 213 conducts, thus 24 maintaining the voltage at the point 207 at the value of the voltage set in by the source 214.

In order to adjust the minimum output voltage from the function generator regardless of the input voltage, a low-limit control is provided. The low-limit control comprises a source of variable direct voltage 216 and a fixed resistor 217 connected in series through a rectifier element 218 to the juncture of rectifier elements 215 and 213. The output voltage from the function generator may be taken from a voltage divider comprising resistances 220, 221, and 222 connected in series between the top of resistor 217 and ground, with a point between the resistances 220 and 221 connected to one terminal of the pair of output terminals 223 and 224, the other output terminal being grounded. It is apparent that, when the voltage present at a point 225 between the rectifier elements 215 and 213 is less than the voltage set in at the variable voltage source 216, the rectifier element 218 does not conduct. However, the variable voltage source 216 causes current to flow through the resistances 217, 220, 221, and 222 and an output signal of fixed value appears at the output terminals 223, 224. When the voltage at point 225 becomes greater than that set in at the variable voltage source 216, rectifier element 218 conducts and the output voltage then varies in the manner previously described.

One further control, known as the overload cut in control, is provided to cause the output voltage to rise more rapidly than is determined by the slope resistors 204b, 205b, 2045b once the input voltage has reached a predetermined level. The overload cut in control comprises resistors 226 and 227 connected in series between input terminal 201 and a source of variable direct voltage 228 with the positive side of the variable voltage source 220 being grounded. A point 230 between resistors 226 and 227 is connected to the point 225 through a rectifier element 231 arranged to pass current from the point 231 toward the point 225. The voltage at point 230 does not affect the voltage at point 225 until it becomes greater than the voltage at point 207. The point 230 is negative until the input voltage connected between terminal 200 and 201 is equal to or greater than the voltage set in at the variable voltage source 228. As the input voltage increases beyond that point, the voltage at point 230 rises at a proportional rate until it reaches the value of the voltage at point 207. At that time, the rectifier element 231 conducts and the rectifier element 215, acting much like a switch, ceases to conduct. Any further increase in the input voltage causes the output voltage to increase by a factor determined by the voltage divider composed of resistors 226 and 227 rather than by the voltage divider composed of resistors 202 and 203 and their associated arms 204, 205, 206.

Referring now to Fig. 7, the efiects of the various controls can be seen. As the input voltage increases from Zero, the output voltage is constant at a predetermined value adjusted by the low-limit control. When the input voltage reaches a point 232, where the voltage at the point 207 (Fig. 5) equals the low-limit voltage as determined by the setting of the variable voltage source 216, the output voltage starts to rise with a slope determined by the ratio of the fixed resistors involved. The points 233, 234, and 235 are those at which the various resistance arms 204, 205, 206 are cut into the circuit and placed in parallel with the resistor 203, as previously described. When the voltage at the point 207 has reached another predetermined value, as at 236, the overload cut-in be comes operative and the slope of the input-output curve again changes. Finally, when the input voltage has reached a predetermined level, as at 237, the high-limit control becomes operative, the rectifier element 213 conducts, and the output voltage is held at a predetermined value.

Although the output of the function generator follows a curve that is actually a series of straight lines of vari- 25 ous slopes, such an approximation has been found to be sufficiently accurate for the purpose. If a closer approximation is desired, it may be obtained by providing additional intercept arms similar to the arms 204, 205, and 206, to provide a greater number of straight line portions for the curve.

It is apparent to one skilled in the art that the variable voltage sources 2040, 205e, 206a, 214, 216, and 228 may be adjustable taps on the output of a conventional regulated direct current power supply. It is further apparent that the diagram shown in Fig. is simplified and that various refinements may be added by one skilled in the art.

It will be recalled that the direct voltage output of each function generator 74 is supplied to one input of a balancing amplifier 75. Each balancing amplifier 75 serves to compare the output signal of the function generator with the signal that is proportional to the output of the corresponding controlled generator or, alternatively, proportional to the valve position of the turbine driving the generator. The balancing amplifier then pro duces a signal to cause a motor to rotate at a speed proportional to the difference between the two input signals and in a direction determined by the polarity of the difference. Although the present invention is not limited to the use of any particular balancing amplifier, one which is known to be suitable is shown in schematic form in Fig. 8.

As is seen from Fig. 8, the balancing amplifier 75 is quite similar to the tie-line load controller 20 previously described with reference to Fig. 3 and differs from the tie-line load controller primarily in that the tachometer feedback and the reference signal producing means are eliminated. Reference is also made to previously mentioned application Serial No. 395,117 for a detailed explanation of the balancing amplifier 75.

Referring now to Fig. 8, one of the direct voltage input signals may be supplied between terminals 240 and 241 and the other direct voltage input signal between terminals 242 and 243. As shown, the input signals should be supplied with the same polarities to the terminals 241 and 242, because the terminals 240 and 243 are grounded.

The terminals 241 and 242 are respectively connected to the two fixed contacts 244a, 244b of a conventional chopper 244 having a movable or vibrating contact 2440. The energizing coil 244d of the chopper may be connected to a secondary winding of a transformer 245 whose primary winding is connected to the system power lines. Thus, as is well known to one skilled in the art, the movable contact 2440 of the chopper is switched between the fixed contacts 244a, 244b at the frequency of the voltage that energizes the operating winding 244d. The movable contact 244a of the chopper is connected through a coupling capacitor 246 to the control grid of the first stage of a conventional three-stage amplifier 247 comprising triode discharge devices 247a, 247b, 247c.

It is now apparent that, during one half of each cycle of the energizing alternating current supplied to the operating winding 244d of the chopper, the signal supplied to the control grid of the first stage 247a of the amplifier 247 is that signal supplied between the input terminals 240 and 241, while during the other half cycle the signal supplied between terminals 242 and 243 is supplied to the amplifier. Thus, whenever one of the input signals is greater than zero and the two signals are not equal, an alternating current square-wave signal will be present on the control grid of the first stage 247a of the amplifier. Of course, the amplitude and phase of that alternating current square-wave signal depend on the difference in amplitude between the two direct current input signals and the polarity of that difference.

Anode voltages may be supplied to the three-stage amplifier 247 by a half-wave rectifier 250 energized from a secondary winding on transformer 245. The direct voltage output of the rectifier 250 is filtered by a conventional resistance-capacitance filter network 251 before being supplied to the anodes of the amplifier stages 247a, 247b, 2470.

The square-wave output signal from the third stage 2470 of amplifier 247 is connected through a coupling capacitor 252 to the control grids of a pair of duo-triode electron discharge devices 253 and 254 provided with a grid resistor 255. As in the tie-line load controller previously described, the anodes of the duo-triode 253 are connected together to one end of a secondary winding on transformer 245 and the anodes of duo-triode 254 are connected together to the other end of that secondary winding. The cathodes of the duo-triodes 253 and 254 are provided with a common cathode resistor 256, and the anode-cathode circuit is completed through a capacitor 257 to a center tap on the transformer secondary winding to which the anodes of the duo-triodes 253 and 254 are connected. A pair of output terminals 258 and 260 are provided on opposite sides of the capacitor 257.

As is apparent from the description of the tie-line load controller previously considered, the alternating current signal supplied to the control grids of the duo-triodes 253 and 254 is either in phase with the anode voltage of duotriode 253 and out of phase with the anode voltage of duo-triode 254, or vice versa. Thus, depending on the phase of the alternating current signal, one of the duo-triodes conducts more heavily than the other; and if one phase of a reversible two-phase motor, such as the motor 66 in the load-proportioning unit (Fig. 2), is connected between the output terminals 258 and 260, and the other phase is connected to the source that energizes the primary winding of transformer 245, there is greater current flow through the motor winding during one half cycle than during the other half cycle to cause the motor to rotate in one direction or the other. Of course, the direction in which the motor rotates is determined by the polarity of the difference between the input signals connected between terminals 240 and 241 and between terminals 242 and 243; and its speed of rotation is related to the amplitude of that diiference. When there is no difference between the input signals or when there are no input signals, equal currents flow through the anode-cathode circuit of the duo-triodes during both half cycles and the motor does not rotate;

It was previously mentioned that the electronic positioning unit 76 (Fig. 2) serves to compare the phase or frequencies of two alternating current signals and, if there is a phase difference, energize one or another output circuit depending on the polarity of the phase difference. Although the invention is not limited to the use of any particular device for this purpose, one such device forms the subject matter of the copending application previously noted and is shown schematically in Fig. 9 of the present application.

Referring now to Fig. 9, the signals whose phases are to be compared, which may be taken from the rotor of selsyn 65 and from the rotor of selsyn 77 (Fig. 2), may be supplied between input terminals 265 and 266 and between input terminals 267 and 268, respectively. The signal supplied between terminals 265 and 266 will be hereafter referred to as signal A," while that signal supplied between terminals 267 and 268 will be hereafter referred to as signal B.

Signal A supplied between terminals 265 and 266 appears across resistors 270 and 271 of equal value connected in series between the terminals. The juncture of the resistors 270 and 271 is grounded, and it is apparent that the signals appearing across these resistors are of equal amplitude but of opposite polarity. The signal appearing across resistor 270 is connected to the control grid of a triode electron discharge device 272, and the signal appearing across resistor 271 is similarly connected to the control grid of a triode electron discharge device 273. The cathodes of the two triodes are connected together and grounded, and their anodes are connected 

